A number of years ago, I researched a number of various sales outlets for an item available for use to cut or saw the rectangular hole available for electrical contractors to install the boxes for light switches and receptacles in building construction. Not having found any, I proceeded to build one myself.
To start, a hard wood model was made. It proved only the cam operating the pair of blades was feasible, in theory. Next, a sandwiched construction of sheet metal of reasonable thickness failed when pressure was applied to penetrate the work piece, plaster dusts froze the mechanism and the sleeve bearings on the shaft. The vibration caused by the reciprocating, motion forced the steel balls to fall away. The steel balls were added to help eliminate the friction and sticking.
Avoiding the failures and using the successes in the third design I finally reached the wall with the blades, following the entry of the single pilot drill being used as a stabilizer. However, this design failed because the drill, which was rotating, immediately end-milled its hole to a larger diameter, because the saw blades touched and dug into the plaster and remained stationary. This proved the need to avoid the kickback by maintaining a stationery position of the body of the device to enable the saw blades to operate correctly.
Finally, two drills were used to stabilize the unit, and they were not rotating when the saw blades were engaged in the work piece. Experiments showed it was necessary to maintain a stationary position first to enable the saw blades to operate successfully. Swing arms replaced the use of steel balls for anti-friction purposes. The position of the swing arms as attached to the lower cover of the gearbox is critical; they must be to an offset position to create the downward thrust for the chopping action along with the sawing action allowing the device to move swiftly through the material.
Along the way, the introduction of rocker arms to operate the short saw blades was a success, no attempt was made to design a formed circular cam since this saw is for "rough cutting of material". Therefore, the eccentric and sealed bearing provided the force necessary to move four saw blades at once.
The rectangular hole size is selected by checking sizes of most boxes now offered on the market. However, by use of shims or moving of the saw blade position and the type of saw teeth custom fitting can be obtained.